The Arabic verb “to be” is كانَ (kāna), which makes it a “hollow verb” (the second letter is a vowel, not a consonant). These have a slightly irregular conjugation, where the long vowel may disappear in the past tense:

• “I was” = كُنتُ (kuntu)
• “you (m, sing.) were” = كُنتَ (kunta)
• “you (f, sing.) were” = كُنتِ (kunti)
• “he was” = كانَ (kāna)
• “she was” = كانًت (kānat)
• “you (dual) were” = كُنتُما (kuntumā)
• “they (m, dual) were” = كُنا (kunā)
• “they (f, dual) were” = كُنتا (kuntā)
• “we were” = كُنّا (kunnā)
• “you (m, pl.) were” = كُنتُم (kuntum)
• “you (f, pl.) were” = كُنتُنَّ (kuntunna)
• “they (m) were” = كانوا (kānū)
• “they (f) were” = كُنَّ (kunna)

In the present tense the verb is usually omitted. Simple subject-predicate sentences are identified when the subject and the predicate fail to agree on definite-indefinite, like “the teacher is a man”: المُدَرِّس رَجُلٌ (al-mudarris rajulun). An indefinite noun/adjective does not take the definite marker ال, and its final short vowel is doubled (though this has the effect of adding a final “-n” sound (be aware that when speaking the language these final short vowels and endings usually just disappear. If the subject and the predicate are both definite, then the appropriate (in terms of gender/number) third person pronoun may be inserted between them to make it clear that this is a sentence, so “the teacher is that man”: المُدَرِّس ذٰلِك الرَجُلُ (al-mudarris dhālik al-rajul).

HOWEVER, we should know the present tense form of كانَ for other reasons, so let’s conjugate it:

• “I am” = أكونُ (akūnu)
• “you (m, sing.) are” = تَكونُ (takūnu)
• “you (f, sing.) are” = تَكونينَ (takūnīna)
• “he is” = يَكونُ (yakūnu)
• “she is” = تَكونُ (takūnu)
• “you (dual) are” = تَكونانِ (takūnāni)
• “they (m, dual) are” = يَكونانِ (yakūnāni)
• “they (f, dual) are” = تَكونانِ (takūnāni)
• “we are” = نَكونُ (nakūnu)
• “you (m, pl.) are” = تَكونونَ (takūnūna)
• “you (f, pl.) are” = تَكُنَّ (takunna)
• “they (m) are” = يَكونونَ (yakūnūna)
• “they (f) are” = يَكُنَّ (yakunna)

One reason it’s good to know the present tense of كانَ is because negating the past tense in Arabic, paradoxically, requires using the present tense form of the verb, in the “jussive” mood. You can compare the conjugation below with the one above to see the differences in the jussive as compared with the simple present tense. Preceding the verb is the negative particle لَم (lam):

• “I was not” = لَم أكُن (lam akun)
• “you (m, sing.) were not” = لَم تَكُن (lam takun)
• “you (f, sing.) were not” = لَم تَكوني (lam takūnī)
• “he was not” = لَم يَكُن (lam yakun)
• “she was not” = لَم تَكُن (lam takun)
• “you (dual) were not” = لَم تَكونا (lam takūnā)
• “they (m, dual) were not” = لَم يَكونا (lam yakūnā)
• “they (f, dual) were not” = لَم تَكونا (lam takūnā)
• “we were not” = لَم نَكُن (lam nakun)
• “you (m, pl.) were not” = لَم تَكونوا (lam takūnū)
• “you (f, pl.) were not” = لَم تَكُنَّ (lam takunna)
• “they (m) were not” = لَم يَكونوا (lam yakūnū)
• “they (f) were not” = لَم يَكُنَّ (lam yakunna)

‌Negating the present tense of كانَ requires a completely irregular form:

• “I am not” = لَستُ (lastu)
• “you (m, sing.) are not” = لَستَ (lasta)
• “you (f, sing.) are not” = لَستِ (lasti)
• “he is not” = لَيسَ (laysa)
• “she is not” = لَيسَت (laysat)
• “you (dual) are not” = لَستُما (lastumā)
• “they (m, dual) are not” = لَيسا (laysā)
• “they (f, dual) are not” = لَيسَتا (laysatā)
• “we are not” = لَسنا (lasnā)
• “you (m, pl.) are not” = لَستُم (lastum)
• “you (f, pl.) are not” = لَستُنَّ (lastunna)
• “they (m) are not” = لَيسوا (laysū)
• “they (f) are not” = لَسنَ (lasna)